1. Field of the Invention
The invention relates to a method for the removal of water and aqueous liquids from the sump of an underground cavity, especially a cavern.
2. The Prior Art
Underground cavities, e.g. caverns located in salt domes, can be used for storage of large amounts of gases, liquids and other substances to be stored
Caverns are produced in a salt dome by solution mining. For this, fresh water is continuously sent into the salt dome, under pressure, through a bore hole, with the desired cavity forming as the salts dissolve and the resulting brine is flushed out. In order to be able to utilize the cavern volume formed by solution mining for storage, the brine is removed from the cavern after solution mining has been stopped. With the method used for creating a gas cavern, a sump remains at the bottom of the cavern the first time the cavern is filled with gas. This sump essentially consists of the aqueous salt solution, and its surface and volume are essentially determined by the geometric dimensions of the cavern and the installation depth of the brine removal equipment.
When placing a gaseous medium, e.g. natural gas, in storage, the gas comes into contact with the free surface of the sump and absorbs water, i.e. water vapor. During subsequent removal of the gas from storage, hydrate formation can occur in a bore hole or subsequent gas distribution network, as a result of the water or water vapor content of the gas. In order to prevent this, the removal rate has to be reduced, in order to prevent cooling, which occurs due to the Joule-Thomson effect based upon adiabatic expansion of the gas, to such an extent that the temperature does not drop below the water vapor dew point. This is undesirable, since the stored gas is particularly needed to cover peak demand over short periods of time with high consumption rates. In any case, the gas taken from storage in caverns must be dried, in a complicated technical process, before being introduced into the gas distribution network which serves to transport the gas to the consumers.
Furthermore, the quality of the stored gas can be detrimentally affected by the moisture absorbed. For example, the occurrence of an increased hydrogen sulfide content in natural gas after being removed from caverns is attributable to the hydrolysis of carbonyl sulfide which occurs in many types of natural gas.
In order to reduce the expenditure for such drying processes, it has already been attempted to prevent absorption of water or water vapor by the gaseous medium, e.g. natural gas, stored in the cavern, by covering the sump in the cavern with a material which inhibits or, if possibly, completely prevents material transport of water from the sump into the stored gas.
A disadvantage of the use of cover layers in the solid aggregate state is that because of the lack of self-sealing properties, the blockage effect is eliminated by crack formation or destruction, for example if chunks of stone or salt come loose from the walls of the cavern or cavity and fall onto it.
Liquid or plastic cover layers do demonstrate self-sealing properties, but these are not suitable for preventing thermodynamic equilibrium from occurring in a subterranean cavity. Such cover layers can slow the diffusion of water from the sump into the medium stored in the cavity, but cannot prevent it completely.